1. Field of the Invention
The invention relates to drive mechanisms for controlling vertically movable bars in liquid-cooled nuclear reactors. It particularly relates to a mechanism comprising a rod supporting the bar and movable vertically in the fixed internal equipment of the reactor and a device for absorbing the shock of the fall of the rod, with hydraulic cylinder and a piston. It is particularly, although not exclusively, for use in pressurized water moderated and cooled reactors.
2. Prior Art
Drive mechanisms of the above-defined type have been used in nuclear reactors. The bar is generally formed of a "spider" with arms radiating from a pommel and absorbent rods fixed to the arms and containing neutronic poison. Movement of the bars makes it possible to control the reactivity in the core of the reactor. For an emergency shutdown of the reactor, all bars are inserted simultaneously by releasing their drive mechanisms so that the bars fall under the action of their own weight. The purpose of the shock absorbing device is to dampen the shock when the pommel comes into abutment against the upper core plate of the reactor or against the upper end piece of the fuel assembly with which the bar is associated.
Most shock absorbing devices have insufficient efficiency and/or oppose an excessive pressure loss to the flow of the cooling fluid. Referring to EP-A-159,509 for example, a control bar has a shock absorbing device consisting of a cylinder formed in the pommel and in which slides a piston biased into a downward projecting position by resilient means contained in the cylinder. Braking by compression of the spring and throttling of the liquid driven out of the cylinder through the clearance between the piston and cylinder varies during the stroke, which adversely affects the efficiency; the volume of the shock absorber must remain small, which makes it impossible to give it a long stroke.